Recombinant adenovirus vectors are powerful tools for inducing de novo gene expression in vivo. Here we have exploited them to study the specificity of CD4/CD8 lineage commitment during thymocyte positive selection, transferring MHC class II genes directly into thymi of mice deficient in both class I and II molecules. Expression of class II molecules was induced on cortical stroma, provoking the selection of a large population of mature CD4(+)CD8(-) cells, as expected, but also of a significant number of CD4(-)CD8(+) cells. The latter constituted a diverse population, containing both immature precursors and, though less frequent, cells that were mature according to several criteria. CD4(-)CD8(+) cells appeared with the same kinetics as their CD4(+)CD8(-) counterparts, but tended to be more prevalent at early times or when thymocyte reconstitution was only modest. These observations, derived from a dynamic selection system, indicate that CD4/CD8 lineage commitment is not irredeemably linked to the class of MHC molecule driving positive selection, a conclusion most compatible with selective models of commitment.